The present invention relates to artificial disc replacement devices. Previous attempts at artificial disc replacement have not received wide spread acceptance because of a number of problems. In some attempts at disc replacement, a flexible artificial disc is placed within the intervertebral disc space without any anchoring system, with the expectation that the artificial disc will remain in place in the disc space based on contact with the ligaments of the disc annulus and/or the vertebral bodies. With this approach there remains an unacceptable rate of protrusion of the artificial disc from the disc space. Further, over time the artificial disc may wear against the adjacent vertebral endplates, generating wear particles in the disc space and creating the risk of failure of the artificial disc.
Alternative designs have provided a rigid interface between the vertebral end plates and a shock absorbing compound disposed in the disc space between the rigid interfaces. The drawbacks of many of these prior devices are that they require extensive disc space preparation prior to placement. Other attempts at disc replacement provide a device having multiple components that must be positioned in the disc space.
The present invention is directed to providing improved artificial disc replacement implants directed to solving a number of the problems and disadvantages of the previous attempts at disc replacement.
The present invention provides an improved artificial disc implant for replacing the spinal disc between two vertebrae of the spine. The implant comprises an upper shell, a lower shell and a spacer or insert therebetween. Preferably the implant is insertable as a single unit into the disc apace between two adjacent vertebral bodies. The shells may be made from any suitable bio-compatible material.
According to one aspect of the invention, the upper and lower shells each include a pair of interconnected cylindrical lobes. In one form, the upper and lower shells are partially cylindrical.
Further, the present invention contemplates insertion into the disc space via tubular instruments presently used for interbody fusion procedures. Thus, the instrumentation utilized to perform current interbody fusion techniques may also serve a dual function for disc replacement procedures.
In another aspect of the present invention, there is provided an upper and lower shell for engagement with the vertebral bone of the adjacent vertebral bodies. The upper and lower shells each have anchoring means to prevent movement in at least one direction. In one form, the anchoring means are ribs that prevent rotation of the shell in the disc space. In another form, each shell includes a flange extending therefrom. Each flange has an aperture extending therethrough receiving a bone screw to engage the shell to the adjacent vertebra.
Another aspect of the present invention, there is provided mating surfaces between the upper and lower shells to restrict the transmission of shear forces through the spacer disposed between the upper and lower shells. In one form, the mating surfaces are provided by multiple projections and that are positionable in corresponding recesses to restrict movement in multiple directions while permitting compression of the spacer disposed between the upper and lower shells.
In still a further aspect of the present invention, shells for contacting the upper and lower vertebral end plates are provided to anchor the device, and multiple spacer shapes are provided between the shells to permit insertion from a variety of approaches to the disc space, including anterior, posterior, lateral, anterior-lateral, and posterior-lateral approaches. The multiple spacer shapes are configured to address a variety of angulations between the adjacent vertebrae. Various instruments and methods for insertion one or more artificial disc implants to the disc space from a variety approaches are also provided.
In yet another aspect of the present invention, the spacer includes two interconnected cylindrical portions and the upper and lower shells have cavities shaped to securely retain the spacer therebetween.
In a further aspect of the invention, the spacer is inserted between the upper and lower shells. The sidewalls of the spacer are truncated adjacent the gap between the upper and lower shells to limit potential impingement of the material between the upper and lower shells as the spacer is compressed.
In yet another aspect of the preferred invention, each of the upper and lower shells includes a substantially cylindrical bone engagement surface for contact with a substantially cylindrical bone opening in the vertebral end plates. Preferably, the shells include structure to engage the bone beyond the opening to limit movement of the shells in at least one direction.
According to another aspect of the present invention, the implant includes a spacer formed from a hydrogel substance. In one method of inserting the implant according to the present invention, the hydrogel is at least slightly dehydrated thereby reducing the height of the implant for insertion. Once inserted, the hydrogel can be hydrated to increase the overall height of the implant to the desired working height.
These and other aspects, features, embodiments, forms, and advantages of the invention will become apparent from the following description of the illustrated embodiments.